While not enforced, the IISS has been using a system of quadrants (four subsectors each) to further subdivide astrographic sectors. Terran Greek lettering are used to mark these quadrants: Alpha, Beta, Delta, and Gamma.

This terminology was developed after many of the core sectors of the Third Imperium were already established and in these areas, it is far more common to hear subsectors referred to by their associated duchy.

The mighty Ziru Sirka originated the sector astrography system. As a giant interstellar empire considerably more advanced than its neighbors, it conquered much of Charted Space. What the Romans are to Terran history is what the ancient Vilani are to the history of Charted Space.

They came, saw, and conquered. And their subjective partitions of space became the standards for astrography. When non-Vilani traders adopted the system, it truly became cemented as the most widely adopted system of interstellar astrography used within Charted Space.

Not all interstellar polities use the Vilani sector system... internally, but they all provide traders with sector compatible star charts to facilitate trade. Even some of the newest races to Charted Space have quickly adopted the system. Profit is a powerful motivator. Even the Terrans who conquered the Ziru Sirka were forced to adopt this Vilani astrographic adaptation.